The present invention is generally directed to an improved printhead for an ink jet printer. More particularly, the invention is directed toward the use of diamond-like-carbon (DLC) to improve the energy efficiency of an ink jet printhead and to protect the relatively delicate thin film resistors of the printhead from corrosive inks and cavitation damage.
A thermal ink jet printer forms an image on a printing surface by ejecting small droplets of ink from an array of nozzles on an ink jet printhead as the printhead traverses the print medium. The ink droplets are formed when ink in contact with a thin film resistive heating element is nucleated due to the heat produced when a pulse of electrical current flows through the heating element. The vaporization of a small portion of the ink creates a rapid pressure increase that expels a drop of ink from a nozzle positioned over the resistive heating element. Typically, there is one resistive heating element corresponding to each nozzle of the array. The resistive heating elements are activated under the control of a microprocessor in the printer electronics of the ink jet printer.
Electrical pulses applied to the heating elements must be sufficient to vaporize the ink. Any energy produced by the resistive heating element of an ink jet printer that is not absorbed by the ink ends up being absorbed by the heater chip. Hence, the total energy applied to the heating element includes the energy absorbed by the chip. This excess energy may result in an undesirable and potentially damaging overheating of the printhead if it is not properly dissipated. Furthermore, because it is desirable to produce an image as quickly as possible, there is a continual push in the ink jet printer industry to increase the number of drops expelled per unit of time. Unfortunately, as the number of nozzle fires in any given amount of time increases, the heat that must be dissipated by the printhead heater chip increases. If the printhead heater chip becomes too hot, the delicate semiconductor structures in the chip may be damaged. Therefore, it is desirable to transfer heat from the resistive element to the ink as efficiently as possible.
Cavitation is another phenomena that may adversely affect the performance of an ink jet print head. Cavitation occurs when, after an ink droplet has been expelled, the ink bubble forcefully collapses back down upon the resistive heating element. This impact can result in a large amount of stress being placed on the surface of the resistive heating element. In fact, this cavitation is so strong that it may actually crack or pit the surface of the resistive heating element and cause it to malfunction. In addition to the cavitation problem, many of the inks used by ink jet printer""s are corrosive. Typically, corrosion resistant passivation layers are used to isolate the heating elements used to eject the droplets of ink from the ink. Unfortunately, these passivation layers reduce the efficiency with which heat is transferred from the heating element to the ink. In addition, the application of a passivation layer increases the number of manufacturing steps required to produce a heating element. Furthermore, the passivation layer may not bond properly to the underlying structures and break loose from the heating element. Thus, prior art heating elements suffer from both passivation and cavitation associated problems that tend to damage the resistive heating elements over time.
Therefore, a need exists for an ink jet printhead that has durable resistive heating elements that more efficiently transfer energy from the heating element to the ink during a printing operation.
The foregoing and other needs are met by a printhead for an ink jet printer having a heating element on a semiconductor chip. The heating element expels droplets of ink from a nozzle on a nozzle plate that is attached to the chip by vaporizing a volume of ink in contact with a surface of the chip. The heating element includes a resistive heating element that increases in temperature and vaporizes the volume of ink when a voltage is applied to the resistive heating element. A diamond-like-carbon (DLC) island is positioned over the resistive heating element. The DLC island is substantially surrounded by a material, such as aluminum, that has a lower thermal conductivity than the DLC island.
The above described embodiment improves upon the prior art in a number of respects. First, by replacing both the cavitation and passivation layers of prior art ink jet heating elements with a single layer of DLC, the invention takes advantage of the exceptionally hard and inert nature of DLC and requires less steps to manufacture. In addition, by surrounding the DLC with a material that has a lower thermal conductivity than DLC, the present invention lowers the energy consumption of the heating element by reducing heat dissipation to the area surrounding the chip and, thus, minimizes the problems associated with over heating of the chip. Furthermore, in the preferred embodiment, a smoothing layer of tantalum insures that nucleation of the ink occurs at the superheat limit.
In another aspect, the invention provides an apparatus for expelling droplets of ink onto a printing surface. The apparatus includes a semiconductor substrate having a first insulating layer deposited over the substrate. A thin resistive heating layer is then deposited over the first insulating layer. A metal conductor layer is deposited over the thin resistive heating layer and a portion of the metal conductor is removed to expose a portion of the thin resistive heating layer. A DLC island is deposited over the exposed portion of the thin resistive heating layer such that the outside perimeter of the DLC island partially overlaps the metal conductor layer. Finally, a second insulating layer is deposited over the metal conductor layer and a portion of the second insulating layer is removed such that all of the metal conductor layer and the outside perimeter of the DLC island are covered by the second insulating layer. This second insulating layer is preferably constructed from an intermetallic dielectric material (IMD). Such IMD materials include but are not limited to silicon nitride, silicon oxide, spun on glass and combinations thereof. A particularly preferred IMD is silicon oxide/spun on glass/silicon oxide.
The DLC island of the above discussed embodiment provides the previously discussed advantages of having a DLC passivation and cavitation protection layer. In addition, the second insulating layer protects the metal conductors from the corrosive effects of the ink and prevents current from leaking from the conducting layer into the ink. Thus, the invention substantially improves upon the prior art ink ejecting devices.
In yet another aspect, the invention provides a heater for expelling ink from a nozzle of an ink jet printer. The heater includes a DLC island deposited thereon. The DLC island is substantially surrounded with a material that has a lower thermal conductivity than the DLC island. A surface portion of the DLC island that comes into contact with the ink is doped with boron to provide a resistive heating portion. Metal contact portions apply a predetermined voltage to the doped surface portion of the DLC island such that a volume of ink in contact with the surface portion is vaporized.
Constructing the resistive heating portion of a heater out of a doped portion of the DLC island decreases the number of manufacturing steps required to construct a heater for an ink jet printer. In addition, the use of DLC provides the cavitation and passivation advantages of DLC previously discussed. Similarly, the surrounding of the DLC island with a material that has a lower thermal conductivity than DLC decreases the energy required to eject a droplet of ink by reducing the amount of heat dissipating laterally from the perimeter of the heater. Therefore, a number of advantages over the prior art are provided by the present invention.